Tony, Gus' information is almost complete, let me fill in the blanks...
I don't know which system you have, but on Windows Vista 64-bit or Windows 7 64-bit, the default path for the standard 32-bit version of VLC is:
"C:\Program Files (x86)\VideoLAN\VLC"
Also, in DOS, to move to another directory, you need to add the "change directory" command (cd), so on the command prompt, you type the following (including the quotes), obviously the path might be different depending on your system and vlc version
cd "C:\Program Files (x86)\VideoLAN\VLC"
Once you are on vlc's folder, you can call vlc with options. Gus' list of parameters did not work for me, maybe it has something to do with the version of vlc. Here's one that works perfectly on VLC media player 2.0.3:
vlc videofile.avi --video-filter=scene --vout=dummy --start-time=25 --stop-time=30 --scene-format=png --scene-ratio=1 --scene-path=c:\images\ vlc://quit
On this first example, the file name is "videofile.avi" and I copied the file to the vlc folder for clarity. Also, I created a folder called "images" on the root of the C hard drive. This code will create a bunch of PNG files from second 25 to second 30 (like Gus said, some are empty, this might have something to do with vlc buffering). The "scene-ratio" is the ratio of frames to export (in this example, all frames are exported).
Here's another example, for clarity:
vlc d:\videos\videofile.avi --video-filter=scene --vout=dummy --start-time=5 --stop-time=10 --scene-format=png --scene-ratio=3 --scene-path=c:\images\ vlc://quit
On this second example, the file name is "videofile.avi" and it is in a folder called "videos" on the D hard drive. Again, the path for the exported images is a folder on the root of the C hard drive. This code will create a bunch of PNG files from second 5 to second 10 (like Gus said, some are empty). The "scene-ratio" in this example will export every third image only, but the file names remain the same, so technically, some files are missing.
More information about the vlc command line options can be found here: http://wiki.videolan.org/VLC_command-line_help